The document J. T. G. Pierik et al., Electrical and Control Aspects of Offshore Wind Farms II (Erao II), Volume 2: Offshore wind farm case studies, ECN, 2004, Chapter 8, p. 97-118, describes in the context of voltage control at the connection point of a wind park that this control can be achieved in two ways: centralised control, in which one controller gives reactive-power set-points to all wind turbines, or decentralised control, in which each wind turbine has its own controller, based on local parameters (p. 106/107). In decentralised control, there is no central controller, and the wind turbines' local controllers control the voltage at the wind turbines' terminals. The voltage level at the connection point can be calculated, and the calculated connection-point voltage can be controlled (p. 107-109).
CA 2 700 248 A1 (Repower) describes a wind park with voltage regulation. A wind-park controller (referred to as “farm master”) transmits a control signal for the reactive power to the wind turbines. An additional controller is provided at the wind turbines. When the additional controller identifies a voltage dip at the wind turbine's terminals, there is no need for it to wait for the delayed transmission of a new value of the control signal from the wind-park controller. Instead it anticipates the required change in the reactive power. The new value of the control signal from the wind-park controller which arrives later ensures that sufficient steady-state accuracy is reached.
U.S. Pat. No. 7,606,638 B2 (Repower) describes a wind park with a reactive-power adjustment system. A wind-park controller (referred to as a “higher-level regulator”) and wind-turbine controllers (referred to as “lower-level regulators”) are provided. The wind-park controller determines a reference voltage, in order to set a global power coefficient, and transmits it to the wind turbine controllers. The reference voltage for each wind turbine can also be determined individually for each wind turbine to adapt it to the respective connecting line with its electric parameters. This determination of the individual reference voltages is performed centrally by the wind-park controller. At the wind-turbine controllers the individual reference voltage is limited by limiting device if the reference voltage has an excessive value. The reference voltage is locally feedback-controlled by the wind-turbine controllers thereby ensuring that the voltage at the wind turbines corresponds to the individual reference voltages prescribed by the central wind-park controller.
Similar voltage control is known from EP 2175540 A2 which, however, deals with a system of multiple wind parks (“windfarm”) and a control structure with local windfarm controllers and a superordinate windfarm-system controller. Signals to compensate for line drops are provided by the superordinate windfarm-system controller to the local windfarm controllers.